1. Field of the Invention
The present invention concerns a high strength heat treatable 7000 series aluminum alloy suitable to application uses such as usual machinery parts, general purpose products, and transportation equipments for aircrafts, railway vehicles and automobiles. The present invention particularly relates to a high strength heat treatable 7000 series aluminum alloy of excellent corrosion resistance.
2. Description of the Related Art
Heat treatable 7000 series aluminum alloys are precipitation type alloys capable of obtaining high strength by artificial aging after solution heat treatment and hardening and they are generally classified into Al-Zn-Mg-Cu series alloys and Al-Zn-Mg series alloys. Typically, Al-Zn-Mg-Cu series alloys include 7075(Al-5.5Zn-2.5Mg-1.6Cu-0.2Cr), 7050(Al-6.2Zn-2.3Mg-2.3Cu-0.12Zr), 7150(Al-6.4Zn-2.3Mg-2.3Cu-0.12Zr)and 7055(Al-8.0Zn-2.1Mg-2.3Cu-0.17Zr) and Al-Zn-Mg series alloys include 7003(Al-6.3Zn-0.8Mg-0.17Zr).
In a typical production method, slabs or billets manufactured by melt casting are applied with a homogenizing heat treatment and, in a case of extrusion products, for instance, re-heated and hot extruded, applied with a solution heat treatment in an air furnace or the like and then hardening and, optionally, applied with stretching as required. Subsequently, after working them into a shape of final products, they are controlled to a predetermined strength by an artificial aging. Further, also in a case of plate products, they are applied with a homogenizing heat treatment, hot rolling and, optionally, cold rolling and then solution heat treatment in an air furnace or salt bath furnace followed by hardening and, optionally, cold rolling or stretching. Subsequently, after being worked into final products, they are controlled to a predetermined strength by artificial aging. If the working degree is high, both of the extrusion material and plate materials are treated into soft materials in the course of the production steps (classifying mark O), worked into the shape of final products and then applied with a solution heat treatment and hardening
In the heat treatable 7000 series aluminum alloy, the maximum strength is obtained by T6 treatment. In the heat treatable 7075 series aluminum alloy, typical heat treatment conditions according to JIS-W1103 and MIL-6088F are applied as a heat treatment at 120.degree. C. for 24 hr. after applying the solution heat treatment and hardening. However, the corrosion resistance is deteriorated extremely. For example, in a test in accordance with ASTM-G47, SCC stress resistance (in the ST direction) is extremely lowered as 50 N/mm.sup.2 or less. Further, in a test in accordance with ASTM-G34 (EXCO test), exfoliation corrosion resistance is extremely lowered as rank EC-ED.
In order to increase the corrosion resistance, an over-aging treatment collectively referred to in T7 refining is generally adopted. The SCC stress resistance is increased, for example, to 117-172, 242 and 289 N/mm.sup.2 in T76 refining, T74 refining and T73 refining, respectively. Further, the layerous corrosion resistant property is also increased to the level of rank EB, rank EA and P. However, the strength is reduced remarkably, that is reduced by 15 to 30% relative to the strength of T6 refining. That is, they were used while reducing the strength in order to increase the corrosion resistance.
In view of the above, U.S. Pat. No. 3,856,584 proposes a heat treatment method aiming at high strength and high corrosion resistance together. The method is conducted by a three stage heat treatment after solution heat treatment and hardening, in which aging is applied in the first stage, the reversion is applied in the second stage and re-aging is applied in the third stage. Actual heat treatment conditions are as follows: Aging; at 120.degree. C. for 24 hr. (T6 refining), reversion: at 200-260.degree. C. for 7-120 sec, re-aging: at 115-1250.degree. C. (for optional time). However, the reversion time is as short as 7 to 120 sec, and the heat treatment upon reversion treatment is also limited to a bath type heat treatment furnace such as an oil bath. Further, even if an oil bath corresponding to the size of products can be provided, the temperature elevation rate is slow for materials of large thickness and it is impossible to completely conduct appropriate reversion in such a short period of time.
Further, U.S. Pat. No.5,221,377 also proposes this method. It is described for actual conditions of the heat treatment that aging and re-aging are applied at 120.degree. C. for 24 hr, and reversion is applied at a temperature ranging from 182 to 236.degree. C., which is kept for 5 min or more. This can attain a strength of 579 N/mm.sup.2, which is higher by 10% than 7X50-T6. Further, the exfoliation corrosion resistance property corresponds to that of the rank EC-EB, which is comparable with 7X50-T76. However, the time for aging and re-aging treatment before and after reversion treatment are 24 hr. respectively and the total heat treatment time required for the three stage heat treatment is extremely as long as 50 hr. Further, corrosion resistance is around at such a level that the exfoliation corrosion resistant property corresponds to rank EC-EB, and SCC stress resistance is not even described. Still further, 7000 series aluminum alloy to be applied with this method are limited to those containing Zr as the transition element. Moreover, it is not even described and can not be recognized at all what micro-structure can provide such properties.
As described above, the over-aging treatment such as T76, T74 and T73 has been known as the heat treatment for improving the corrosion resistance of 7000 series aluminum alloys. However, the strength is lowered remarkably. In view of the above, it has been proposed a three stage heat treatment comprising aging, reversion and re-aging after the solution heat treatment, and hardening as a heat treatment method of attaining high strength and high corrosion resistance simultaneously, but the reversion time is as short as several tens seconds, which is not industrially practical. Further, although it has been also intended to make the time for the reversion step longer, the exfoliation corrosion resistant property is still as low as about T76 treatment and it is quite unknown for the SCC resistant property. And still further, it has not yet been recognized at all what micro-structure can provide the high strength and high corrosion resistance.
Demand for reducing the thickness and the weight has become increased more and more in recent years in application uses, for example, to transportation equipments such as aircrafts, railway vehicles and automobiles, and general machinery parts. Further, it has been also a strong demand for making those materials scarcely using aluminum alloys (particularly, 7000 series alloys) so far because of SCC worry with aluminum alloys, thereby reducing the weight and, at the same time, making all of the constituent materials with aluminum alloys, still more, improving the recycling performance. For example, aluminum bolts having high strength and high corrosion resistance have been demanded strongly. In view of the above, improvement regarding higher strength, particularly, higher corrosion resistance (SCC stress resistant and exfoliation-corrosion resistant properties) has been demanded for the 7000 series aluminum alloys.